The Coins
Slide any ten coins to the left to make a second group. Then invert those coins. Both the 40 coins in the right group and the 10 coins in the left group will now show the same number of heads. For if N heads were shifted to the left, then 10 - N tails were shifted and also 10 - N heads remained on the right. Inverting all the coins in the left group converts the 10 - N tails to heads, the same as in the right group.